1. Technical Field
This invention applies to gas turbine engines in general, and to bypass air valves in particular.
2. Background Information
Most augmented gas turbine engines include fan, compressor, combustor, turbine, augmentor, and nozzle sections, arranged forward to aft in the engine. Air compressed within the fan and compressor sections travels into the combustor where fuel is added and ignited. The core gas flow (i.e., the air plus the products of combustion) exits the combustor and enters the turbine section, providing power to drive the turbine. The turbine section, in turn, powers the fan and compressor sections. The core gas flow then travels through the augmentor and nozzle, producing thrust to power the aircraft. Under augmented conditions, fuel is selectively added to the core gas flow within the augmentor and combusted to produce additional thrust exiting the nozzle.
The physical operating conditions (e.g., pressure, temperature, and velocity) of the core gas traveling through the engine will vary depending upon the power setting of the engine and the external environment in which the engine is operating. For example, core gas flow traveling through an engine at ground idle will, in most places, be at a lower temperature, pressure, and velocity than the same in an engine under maximum augmentation. The variations in core gas flow operating conditions necessitate cooling mechanisms capable of handling a variety of operating conditions.
To accommodate the spectrum of operating conditions, most augmented gas turbine engines possess the ability to selectively bleed air off of a fan section or a compressor section. The bled air, also referred to as "bypass air", is most often used to cool downstream components such as the combustor, turbine, augmentor, and nozzle, particularly when the engine is operated at high power and/or augmented. Bypass air selectively bled through or passed by structures adjacent the core gas flow path transfers thermal energy away from those structures. Bypass air entering the augmentor also provides additional oxygen for combustion.
The turbine section of the engine generally includes a plurality of stator vanes ("exit guide vanes") disposed aft of the final turbine rotor stage, which extend between an inner radial cone and the outer radial turbine exhaust case. The exit guide vanes orient the flow exiting the turbine rotor stage in an optimum direction for passage through the augmentor and nozzle. The exit guide vanes disposed in the core gas flow path create a flow impediment generally evidenced by a pocket of relatively static core gas (hereinafter referred to as a quiescent pocket) immediately aft of each exit guide vane. The geometry of the quiescent pocket depends upon the geometry of the vane, the vanes orientation relative to the core gas flow, and the velocity of the core gas flow traveling past the vane. In some engines, the exit guide vanes house the augmentor fuel spray bars and an ignition source for igniting the fuel after its introduction into the core gas flow. In those instances, the exit guide vanes are given a geometry that encourages the formation of a quiescent pocket. The flame is initiated and maintained in the quiescent pocket while the core gas flow passes between the circumferentially spaced exit guide vanes. In other engines, fuel spray bars, flame holding bluff bodies, and ignitors are positioned in the core gas flow path aft of the turbine exit guide vanes. An advantage of the using the exit guide vanes as "bluff bodies" to form quiescent pockets is that the need for the downstream fuel spray bars, flame holding bluff bodies, and ignitors is eliminated. A potential disadvantage of the quiescent pockets, however, are the cooling requirements created by high static pressure and temperature within the pockets. An engine having "x" number of turbine exit guide vanes, for example, will have pressure and temperature gradients extending around the circumference of the core gas flow path with "x" number of relative high pressure and temperature zones.
What is needed is a bypass air valve for a gas turbine engine that increases the efficiency of the engine, and one that can operate in an environment having substantial pressure and thermal gradients without adverse consequence.